This invention relates to single source, multiple-order Raman effect amplifiers and to optical communication systems employing such amplifiers.
Optical fiber transmission systems provide the rapid transmission of vast amounts of information. An optical fiber system comprises, in essence, a source of information-carrying optical signals and an optical fiber transmission line for carrying the optical signals. It may further include a receiver for detecting the signals and demodulating the information they carry. The signals are typically within a wavelength range favorable for propagating within silica optical fibers, and preferably comprise a plurality of wavelength-distinct channels within that range.
The optical fibers are thin strands of glass of composition capable of transmitting optical signals over long distances with very low loss. They are small diameter waveguides having a core with a first index of refraction surrounded by a cladding having a second (lower) index. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the fiber with low attenuation. Typical fibers are made of high purity silica with Germania doping in the core to raise its index of refraction above the cladding. A fiber transmission line may include many long segments separated by intermediate nodes for adding or dropping signal channels.
Amplifiers are important components in long distance optical transmission systems. Despite significant progress in reducing attenuation in optical fibers, optical signals transmitted through them are attenuated by the cumulative and combined effects of absorption and scattering. Consequently, long distance transmission requires amplification.
Multiple-order Raman amplifiers are advantageous components to achieve the desired amplification. First order Raman amplification is produced by pump light of higher frequency traveling in the same fiber as the signal light. Multiple-order Raman amplification uses even higher frequency light to amplify the first order pump light that amplifies the signal light. Multiple-order amplification can provide a more favorable distribution of signal-amplifying light along the length of the fiber.
On a more technical level, in first order Raman amplification, signal light in a material medium stimulates higher frequency first order pump light to convert to the lower frequency signal light. This produces a gain in the strength of the signal light.
FIG. 1 schematically illustrates the first order conversion process. A molecule absorbs a photon at frequency fp, and is excited up to a non-resonant (virtual) level. The molecule decays to a lower energy state emitting a signal photon at a lower frequency fs in the process. The energy difference between the pump and the signal photons is dissipated by the molecular vibrations of the medium. The molecular vibration energy levels of the fiber determines the frequency dependence of the gain. FIG. 2 illustrates the spectral gain curve for germania doped silica.
A given signal frequency ƒs in a fiber will be amplified by a higher frequency pump ƒp with the amount of gain dependent on the frequency difference (ƒp-ƒs) called the Stokes shift. The Stokes shift for which ƒs is maximally amplified is called the first order Stokes shift, and significant amplification typically occurs over the range of Stokes shifts between 1/2 the first order shift and 3/2 the first order shift. The first order Stokes shift in Germania-doped silica fibers is about 13 terrahertz (THz).
An advantage of first order Raman amplification over conventional optical amplification is that Raman amplification occurs along a length of transmission fiber rather than at the location of a specific component. This permits amplification to take place well upstream of the pump before the signal has suffered irrecoverable attenuation and distortion. Nonetheless much of the first order pump energy is consumed within a short distance of entering the transmission fiber.
A multiple-order Raman amplifier uses yet higher frequency pump light (e.g., a second order pump) to amplify the first order pump light. FIG. 3 is a qualitative spectral diagram showing how the second order Raman pump can amplify the first order pump which, in turn, amplifies the signal light.
One advantage of adding a second order pump is that one can better control the distribution of first order pump energy along the length of the transmission fiber. This permits enhancing amplification even further upstream from the first order pump source. FIG. 4 is a graphical illustration showing the evolution of signal power in three different Raman amplification arrangements. Curve 1 shows the evolution of signal power with position in a fiber including counter propagating first order pump light. Curve 2 shows the effect of adding co-propagating second order pump light, and curve 3 shows the effect if the second order pump light is counterpropagating. As can be seen, multiple-order pumping enhances amplification away from the pump source. The structure and operation of multiple-order Raman amplifiers are described in greater detail in U.S. Pat. No. 6,163,636 issued to A. J. Stentz, et al. on Dec. 19, 2000, which is incorporated herein by reference.
The cost of light sources is a major portion of the cost of amplifiers in fiber optic communication systems and a significant portion of the cost of the system. While Raman amplifiers have important quality advantages over conventional rare-earth doped fiber amplifiers, rare-earth amplifiers require only one pump source. Multiple-order Raman amplifiers, in contrast, have typically required separate sources for the first and second order pumps. As a consequence, increased cost has been an impediment to expanded use of high quality multiple-order Raman amplification.
In accordance with the invention, a multiple-order Raman amplifier comprises a single source, multiple order Raman pump source, a length of optical fiber and a coupler for injecting the pump light into the fiber. An optical fiber transmission system comprising a source of optical signal channels and an optical fiber transmission line is provided with one or more single source, multiple-order Raman amplifiers. Each single source pump provides multiple-order Raman pump light for amplifying the signal channels.